rollback poison to 3.1.0 back as phoenix will crash with new one

for more info see devinus/poison#172

mix.exs

@@ -54,7 +54,7 @@ defmodule PhoenixSwagger.Mixfile do
   # Type `mix help deps` for more examples and options
   defp deps do
     [
-      {:poison, "~> 2.2 or ~> 3.0 or ~> 4.0.1", optional: true},
+      {:poison, "~> 2.2 or ~> 3.0", optional: true},
       {:jason, "~> 1.0", optional: true},
       {:ex_json_schema, "~> 0.7.1", optional: true},
       {:plug, "~> 1.10"},

mix.lock

@@ -7,6 +7,6 @@
   "mime": {:hex, :mime, "1.3.1", "30ce04ab3175b6ad0bdce0035cba77bba68b813d523d1aac73d9781b4d193cf8", [:mix], [], "hexpm"},
   "plug": {:hex, :plug, "1.10.3", "c9cebe917637d8db0e759039cc106adca069874e1a9034fd6e3fdd427fd3c283", [:mix], [{:mime, "~> 1.0", [hex: :mime, repo: "hexpm", optional: false]}, {:plug_crypto, "~> 1.1.1 or ~> 1.2", [hex: :plug_crypto, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm"},
   "plug_crypto": {:hex, :plug_crypto, "1.1.2", "bdd187572cc26dbd95b87136290425f2b580a116d3fb1f564216918c9730d227", [:mix], [], "hexpm"},
-  "poison": {:hex, :poison, "4.0.1", "bcb755a16fac91cad79bfe9fc3585bb07b9331e50cfe3420a24bcc2d735709ae", [:mix], [], "hexpm"},
+  "poison": {:hex, :poison, "3.1.0", "d9eb636610e096f86f25d9a46f35a9facac35609a7591b3be3326e99a0484665", [:mix], [], "hexpm"},
   "telemetry": {:hex, :telemetry, "0.4.2", "2808c992455e08d6177322f14d3bdb6b625fbcfd233a73505870d8738a2f4599", [:rebar3], [], "hexpm"},
 }